U.S. patent number 7,684,367 [Application Number 12/041,413] was granted by the patent office on 2010-03-23 for apparatus and method for transmitting data using multi-round contention avoidance.
This patent grant is currently assigned to Samsung Electronics Co., Ltd., Seoul National University Industry Foundation. Invention is credited to Jae-Hyun Ahn, Sae-Woong Bahk, Dai-Kwan Kim, Jong-Hyung Kwun, Jeong-Kyun Yun.
United States Patent |
7,684,367 |
Yun , et al. |
March 23, 2010 |
Apparatus and method for transmitting data using multi-round
contention avoidance
Abstract
An apparatus and method for transmitting data by using
Multi-round Contention Avoidance (MrCA) are provided. The method
includes initializing a contention window for each of a plurality
of contention rounds and setting a backoff counter corresponding to
a size of the contention window for each contention round, starting
from a first contention round to a last contention round,
decrementing a backoff counter of a corresponding contention round
in a time slot unit and transmitting data when the decremented
backoff counter is zero and when the corresponding contention round
is a last contention round. Accordingly, the number of contending
nodes is exponentially decreased for each contention round,
resulting in significant decrease in a collision probability in
addition to improving fairness on channel use among users.
Inventors: |
Yun; Jeong-Kyun (Anyang-si,
KR), Bahk; Sae-Woong (Seoul, KR), Ahn;
Jae-Hyun (Seoul, KR), Kim; Dai-Kwan (Seoul,
KR), Kwun; Jong-Hyung (Seongnam-si, KR) |
Assignee: |
Samsung Electronics Co., Ltd.
(Suwon-si, KR)
Seoul National University Industry Foundation (Seoul,
KR)
|
Family
ID: |
39732972 |
Appl.
No.: |
12/041,413 |
Filed: |
March 3, 2008 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20080212477 A1 |
Sep 4, 2008 |
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Foreign Application Priority Data
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Mar 2, 2007 [KR] |
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10-2007-0020777 |
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Current U.S.
Class: |
370/329 |
Current CPC
Class: |
H04L
12/66 (20130101); H04L 43/0888 (20130101) |
Current International
Class: |
H04W
4/00 (20090101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Jae-Hyun Ahn et al., Tone-based Access Scheme with Repetitive
Contention in IEEE 802.11 DCF, JCCI 2006, Apr. 2006. cited by other
.
Jeongkyun Yun et al., Multi-round Collision Avoidance for
Contention-Based Medium Access Control, Institute of Korean
Electronic Association, Nov. 25, 2006. cited by other.
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Primary Examiner: Yao; Kwang B
Assistant Examiner: Loo; Juvena
Attorney, Agent or Firm: Jefferson IP Law, LLP
Claims
What is claimed is:
1. A data transmission method using Multi-round Contention
Avoidance (MrCA) in a contending node, the method comprising:
initializing, by a mobile station, a contention window for each of
a plurality of contention rounds; setting a backoff counter
corresponding to a size of the contention window for each
contention round; starting from a first contention round to a last
contention round, decrementing the backoff counter of a
corresponding contention round in a time slot unit; and
transmitting data when the decremented backoff counter is zero and
the corresponding contention round is a last contention round.
2. The method of claim 1, wherein the backoff counter of each
contention round is randomly set to a value in the range of [0,
CW], where CW denotes a size of the contention window.
3. The method of claim 1, further comprising: if the decremented
backoff counter is not equal to zero, determining whether a tone
signal is received; if the tone signal is received, suspending the
decrementing of the backoff counter until a specific node, which
wins in a current contention, completes the transmission of data;
and if the tone signal is not received, decrementing the backoff
counter of the corresponding contention round in a time slot
unit.
4. The method of claim 3, further comprising, if a channel is not
used for a time period while in the suspended state, releasing the
suspended state and decrementing the backoff counter of the
corresponding contention round in the time slot unit.
5. The method of claim 4, wherein the time period is equal to
2(CW.sub.min+1), where CW.sub.min denotes a minimum size of the
contention window.
6. The method of claim 1, further comprising, when the decremented
backoff counter is equal to zero and when the corresponding
contention round is not the last contention round, transmitting a
tone signal within one time slot and proceeding to a next
contention round to decrement the backoff counter of the
corresponding contention round in the time slot unit.
7. The method of claim 1, further comprising, if a collision occurs
for the transmitted data, increasing the contention window,
starting from the first contention round to a contention round
determined by the number of collisions, and setting the backoff
counter for each contention round by using the contention
window.
8. The method of claim 7, wherein the contention window is
increased by using Equation: .times..A-inverted..times..di-elect
cons.<.ltoreq..A-inverted..times..di-elect cons.<.ltoreq.
##EQU00002## where CW.sub.i denotes a contention window of an
i.sup.th contention round, CW.sub.min denotes a minimum size of the
contention window, j denotes the number of collisions, and G
denotes a last contention round.
9. The method of claim 1, further comprising, if a collision does
not occur for the transmitted data, in order to contend for next
data transmission, initializing the contention window for each
contention round, and setting the backoff counter for each
contention round by using the initialization result.
10. The method of claim 9, wherein, if the collision does not occur
for the transmitted data, in order to contend for the next data
transmission, setting the random backoff counter of the first
contention round to a value in the range of [1, CW+1], and setting
the random backoff counters of the remaining contention rounds to a
value in the range of [0, CW].
11. A data transmission apparatus using Multi-round Contention
Avoidance (MrCA) of a contending node, the apparatus comprising: an
initialization/setup unit for initializing a contention window for
each of a plurality of contention rounds and for setting a backoff
counter corresponding to a size of the contention window for each
contention round; a decrementer for decrementing a backoff counter
of a corresponding contention round in a time slot unit, starting
from a first contention round to a last contention round; a
comparator for determining whether the decremented backoff counter
is zero and whether the corresponding contention round is the last
contention round; and a data transmitter for receiving the backoff
counter which is zero of the last contention round from the
comparator and for transmitting data to a receiving side.
12. The apparatus of claim 11, wherein the initialization/setup
unit randomly sets the backoff counter of each contention round to
a value in the range of [0, CW], where CW denotes a size of the
contention window.
13. The apparatus of claim 11, wherein, when the backoff counter of
the corresponding contention round is not equal to zero, the
comparator determines whether a tone signal is received, and, if
the tone signal is not received, outputs the backoff counter of the
corresponding contention round to the decrementer, and otherwise,
if the tone signal is received, outputs the backoff counter of the
corresponding contention round to the decrementer after suspending
the decrementing until a specific node which wins in a current
contention completes the transmission of data.
14. The apparatus of claim 13, wherein, if a channel is not used
for a time period when in the suspended state, the comparator
releases the suspended state and outputs the backoff counter of the
corresponding contention round to the decrementer.
15. The apparatus of claim 14, wherein the time period is equal to
2(CW.sub.min+1), where CW.sub.min denotes a minimum size of the
contention window.
16. The apparatus of claim 11, wherein, when the backoff counter of
the corresponding contention round is equal to zero and when the
corresponding contention round is not the last contention round,
the comparator transmits a tone signal within one time slot and
outputs the backoff counter of the corresponding contention round
to the decrementer.
17. The apparatus of claim 11, further comprising a collision
detector for detecting a collision of the transmitted data, and, if
the collision occurs, for controlling the initialization/setup unit
to increase the contention window, starting from the first
contention round to a contention round determined by the number of
collisions.
18. The apparatus of claim 17, wherein, if the collision does not
occur, in order to contend for the next data transmission, the
collision detector controls the initialization/setup unit to set
the backoff counter of the first contention round to a value in the
range of [1, CW+1] and to set the backoff counters of the remaining
contention rounds to a value in the range of [0, CW].
19. The apparatus of claim 17, wherein the contention window is
increased by using Equation: .times..A-inverted..times..di-elect
cons.<.ltoreq..A-inverted..times..di-elect cons.<.ltoreq.
##EQU00003## where CW.sub.i denotes a contention window of an
i.sup.th contention round, CW.sub.min denotes a minimum size of the
contention window, j denotes the number of collisions, and G
denotes a last contention round.
20. The apparatus of claim 11, further comprising a tone signal
transceiver for transmitting a tone signal under the control of the
comparator and for outputting a received tone signal to the
comparator.
Description
PRIORITY
This application claims the benefit under 35 U.S.C. .sctn.119(a) of
a Korean patent application filed in the Korean Intellectual
Property Office on Mar. 2, 2007 and assigned Ser. No. 2007-20777,
the entire disclosure of which is hereby incorporated by
reference.
JOINT RESEARCH AGREEMENT
The presently claimed invention was made by or on behalf of the
below listed parties to a joint research agreement. The joint
research agreement was in effect on or before the date the claimed
invention was made and the claimed invention was made as a result
of activities undertaken within the scope of the joint research
agreement. The parties to the joint research agreement are Samsung
Electronics Co. Ltd. and Seoul National University Industry
Foundation.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and method for
transmitting data by using Multi-round Contention Avoidance (MrCA).
More particularly, the present invention relates to an apparatus
and method for transmitting data by using MrCA in a wireless
network employing a wireless random access channel or a
distribution access scheme.
2. Description of the Related Art
Slotted ALOHA is an access scheme generally used in a next
generation wireless access environment that uses a random access
channel. In the slotted ALOHA scheme, when it is desired to
transmit data using a wireless resource, a user attempts to
transmit the data in a time slot immediately subsequent to the
current time slot. If another user is using the same channel, the
two users cannot successfully transmit a frame due to mutual
interference, which leads to retransmission. In retransmission, the
user waits for a random time to prevent the collision from
occurring again, and then transmits the data in a corresponding
slot. However, although the slotted ALOHA scheme has been designed
aiming at simplicity, it has a significant shortcoming in terms of
interference with other users. Specifically, a theoretical
throughput of the slotted ALOHA is e.sup.-1, or approximately
0.368. This means that, in practice, only about 37% of an entire
channel is used in practical transmission under optimal
conditions.
With a throughput of only 37%, it is not possible to provide
services for a system such as a wireless Local Area Network (LAN)
in which a random access scheme is generally used in data
transmission. Therefore, the Institute of Electrical and
Electronics Engineers (IEEE) 802.11 standard has proposed the use
of a Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA) scheme. In the CSMA/CA scheme, before a user attempts to
transmit data through a wireless channel, the user first determines
if another user is using the channel, and, in order to minimize
collisions, waits for a random time before attempting to transmit
the data even if no user is using the channel. In other words, in
the CSMA/CA scheme, when the user has a frame to be transmitted,
the user first determines a state of a wireless channel to be used,
and, if a signal of another user is sensed, waits until the
wireless channel is idle, and otherwise, immediately proceeds to a
collision avoidance process. In the collision avoidance process, a
random time is allocated, and then the user waits for that time
before attempting to transmit data. The CSMA/CA scheme prevents
data from being concurrently transmitted by several users through a
non-busy channel, thereby reducing a collision probability.
However, since the CSMA/CA scheme uses the wireless resource in a
very cautious manner as compared to the slotted ALOHA scheme, the
wireless resource is less effectively used in practice. In
addition, there is a problem in that, even with a small number of
users, time is not efficiently allocated, and with a large number
of users, collision probability and performance deterioration are
both increased. In particular, a collision avoidance function of
the CSMA/CA scheme may cause an unequal distribution of resources,
and thus a large amount of resources may be exclusively used by
some users.
SUMMARY OF THE INVENTION
An aspect of the present invention is to address at least the
above-mentioned problems and/or disadvantages and to provide at
least the advantages described below. Accordingly, an aspect of the
present invention is to provide an apparatus and method for
transmitting data by using Multi-round Contention Avoidance
(MrCA).
Another aspect of the present invention is to provide an apparatus
and method for transmitting data by using MrCA in a wireless
network employing a wireless random access channel or a
distribution access scheme.
Another aspect of the present invention is to provide an apparatus
and method for transmitting data by using MrCA capable of providing
a high throughput in addition to improving fairness among
users.
According to an aspect of the present invention, a data
transmission method using MrCA in a contending node is provided.
The method includes initializing a contention window for each of a
plurality of contention rounds, setting a backoff counter
corresponding to a size of the contention window for each of the
plurality of contention rounds, starting from a first contention
round to a last contention round, decrementing the backoff counter
of a corresponding contention round, in a time slot unit; and
transmitting data when the decremented backoff counter is zero and
the corresponding contention round is a last contention round.
According to another aspect of the present invention, a data
transmission apparatus using MrCA in a contending node is provided.
The apparatus includes an initialization/setup unit for
initializing a contention window for each of a plurality of
contention rounds and for setting a backoff counter for each
contention round corresponding to a size of the contention window,
a decrementer for decrementing the backoff counter of a
corresponding contention round in a time slot unit, starting from a
first contention round to a last contention round, a comparator for
determining whether the decremented backoff counter is zero and
whether the corresponding contention round is the last contention
round and a data transmitter for receiving the backoff counter
which is zero of the last contention round from the comparator and
for transmitting data.
Other aspects, advantages, and salient features of the invention
will become apparent to those skilled in the art from the following
detailed description, which, taken in conjunction with the annexed
drawings, discloses exemplary embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of certain
exemplary embodiments of the present invention will become more
apparent from the following detailed description when taken in
conjunction with the accompanying drawings in which:
FIG. 1 is a block diagram illustrating a structure of an apparatus
for transmitting data by using Multi-round Contention Avoidance
(MrCA) in a Mobile Station (MS) according to an exemplary
embodiment of the present invention;
FIG. 2 is a flowchart illustrating a method of transmitting data by
using MrCA in an MS according to an exemplary embodiment of the
present invention;
FIG. 3 is a diagram for illustrating an MrCA method according to an
exemplary embodiment of the present invention;
FIG. 4 is a graph for comparing a collision rate with respect to
the number of contending nodes according to an exemplary embodiment
of the present invention with a conventional method;
FIG. 5 is a graph for comparing a throughput with respect to the
number of contending nodes according to an exemplary embodiment of
the present invention with a conventional method; and
FIGS. 6A to 6C are graphs for comparing a throughput fairness index
for all nodes according to an exemplary embodiment of the present
invention in comparison with the conventional method.
Throughout the drawings, it should be noted that like reference
numbers are used to depict the same or similar elements, features
and structures.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
The following description with reference to the accompanying
drawings is provided to assist in a comprehensive understanding of
exemplary embodiments of the invention as defined by the claims and
their equivalents. It includes various specific details to assist
in that understanding but these are to be regarded as merely
exemplary. Accordingly, those of ordinary skill in the art will
recognize that various changes and modifications of the embodiments
described herein can be made without departing from the scope and
spirit of the invention. Also, descriptions of well-known functions
and constructions are omitted for clarity and conciseness.
Hereinafter, an exemplary apparatus and method for transmitting
data by using Multi-round Contention Avoidance (MrCA) will be
described.
FIG. 1 is a block diagram illustrating a structure of an apparatus
for transmitting data by using MrCA in a Mobile Station (MS)
according to an exemplary embodiment of the present invention.
Referring to FIG. 1, an exemplary apparatus includes an
initialization/setup unit 100 for initializing a contention window
and for setting a backoff counter, a decrementer 102 for
decrementing the backoff counter, a comparator 104, a tone signal
transceiver 106, a data transmitter 108, and a collision detector
110.
The initialization/setup unit 100 initializes a contention window
for each contention round and sets a backoff counter for each
contention round. In an exemplary implementation, the backoff
counter for each contention round is set to a random value.
The decrementer 102 receives the backoff counter for each
contention round from the initialization/setup unit 100, decrements
the backoff counter of a first contention round, and outputs the
decremented backoff counter to the comparator 104. In an exemplary
implementation, the decrementer decrements the backoff counter in a
time slot unit. The decrementer 102 also receives backoff counters
of the remaining contention rounds from the comparator 104,
decrements a backoff counter of a corresponding contention round in
the time slot unit, and outputs the decremented backoff counter to
the comparator 104.
The comparator 104 determines whether the backoff counter of the
corresponding contention round, which is received from the
decrementer 102, is equal to zero. If the backoff counter of the
corresponding contention round is not zero, the comparator 104
determines whether a tone signal is received from a different MS by
the use of the tone signal transceiver 106. If a tone signal is not
received from a different MS, the comparator 104 outputs the
backoff counter of the corresponding contention round to the
decrementer 102. On the other hand, upon receiving a tone signal
from a different MS, the comparator 104 waits (e.g. in a hold
state) until a specific MS, which wins in a current contention,
completes the transmission of data, and then outputs the backoff
counter of the corresponding contention round to the decrementer
102. If a channel is not used for the transmission of data even
after a time of 2(CW.sub.min+1) (herein, CW.sub.min denotes a
minimum size of the contention window) elapses while in the hold
state, the comparator 104 releases the comparator from the hold
state, and outputs the backoff counter of the corresponding
contention round to the decrementer 102. Meanwhile, if the backoff
counter of the corresponding contention round is zero, the
comparator 104 determines whether the corresponding contention
round is a last contention round. If the corresponding contention
round is not the last contention round, the comparator 104
transmits a tone signal by the use of the tone signal transceiver
106. In an exemplary implementation, the tone signal may be a short
tone signal transmitted within one time slot. If the contention
round is not the last contention round, the comparator 104 outputs
the backoff counter (i.e., 0) of the corresponding contention round
to the decrementer 102 so as to allow the decrementer 102 to
decrement a backoff counter of a next contention round. Otherwise,
if the corresponding contention round is the last contention round,
the comparator 104 outputs the backoff counter (i.e., 0) of the
last contention round to the data transmitter 108.
The tone signal transceiver 106 transmits the tone signal to a
different MS under the control of the comparator 104 or outputs the
tone signal received from a different MS to the comparator 104.
The data transmitter 108 transmits a data signal when the backoff
counter (i.e., 0) of the last contention round is received from the
comparator 104.
The collision detector 110 detects a collision in the transmitted
data signal. If no collision occurs in the transmitted data signal,
the collision detector 110 controls the initialization/setup unit
100 to set a backoff counter for a new contention round in order to
transmit next data. In this case, the backoff counter of the first
contention round is set to a value in the range of [1, CW+1], and
the backoff counters of the remaining contention rounds are set to
a value in the range of [0, CW]. On the other hand, if the
collision occurs in the transmitted data signal, the collision
detector 110 controls the initialization/setup unit 100 to double a
contention window, starting from the first contention round to the
corresponding contention round determined by the number of
collisions, and to set a random backoff counter for each contention
round.
FIG. 2 is a flowchart illustrating a method of transmitting data by
using MrCA in an MS according to an exemplary embodiment of the
present invention.
Referring to FIG. 2, in step 201, the MS initializes a contention
window for each contention round, and sets a backoff counter, which
may be randomly set, for each contention round. That is, if a total
of G contention rounds are to be carried out by a plurality of MSs
to transmit data, G contention windows and G backoff counters are
set for the respective contention rounds. The backoff counter
denotes a value, which may be a random value, allocated for
contention in a corresponding contention round. The contention
window represents a window or time duration during which the
contention is processed and denotes a maximum value that can be set
for the backoff counter. Thus, the backoff counter for each
contention round is set to a value in the range of [0, CW], where
CW denotes the size of the contention window.
In step 203, the MS decrements the backoff counter of a
corresponding contention round and may decrement the backoff
counter in a time slot unit. In the case of a step performed
immediately after step 201, the corresponding contention round is a
first contention round. In step 205, the MS determines whether the
backoff counter of the corresponding contention round is zero. If
the backoff counter of the corresponding contention round is not
zero, the MS determines whether a tone signal is received from a
different MS in step 207. In step 209, upon receiving a tone signal
from a different MS for indicating the termination of counting, the
MS suspends (e.g. puts a hold state) the decrementing of the
backoff counter until a specific MS, which wins in a current
contention, completes the transmission of data, then, the procedure
returns to step 203. In this case, if a channel is not used even
after a time of 2(CW.sub.min+1) (herein, CW.sub.min denotes a
minimum size of the contention window) elapses after stopping the
backoff counting due to the tone signal received from the different
MS, the MS releases the hold state, and, returning to step 203,
continues to decrement the backoff counter. This is to prevent a
system from being halted by an interference signal. On the other
hand, if a tone signal is not received from a different MS,
returning to step 203, the MS repeats the subsequent steps.
If it is determined in step 205 that the backoff counter of the
corresponding contention round is zero, the MS determines whether
the corresponding contention round is the last contention round in
step 211. If the corresponding contention round is not the last
contention round, in step 213, the MS transmits a short tone signal
so as to inform the different MSs that the backoff counting of the
corresponding contention round has stopped. In step 215, the MS
proceeds to a next contention round. Then, returning to step 203,
the MS repeats the subsequent steps. In this case, a plurality of
MSs finish the counting upon receiving the tone signal. However,
MSs which transmit a tone signal proceed to a next contention
round, and continue to participate in the contention. If it is
determined in step 211 that the corresponding contention round is
the last contention round, the MS determines that the MS acquires a
chance to transmit a data signal in the current contention. Thus,
in step 217, the MS transmits the data signal by using a wireless
resource.
In step 219, the MS determines if a collision occurs in the
transmitted data signal. For example, when a Non-ACKnowledge (NACK)
signal is received as a response signal from a receiving side or
when the response signal is not received within a time period, the
MS may determine that a collision occurs in the transmitted data
signal. If no collision occurs in the transmitted data signal,
returning to step 201, the MS initializes the contention window for
each contention round, sets the backoff counter for each contention
round, and repeats the subsequent steps. In order to improve
fairness, that is, in order to prevent an MS, which has acquired a
chance to transmit data in a previous time period, from acquiring
again a chance to transmit data in a current time period, a backoff
counter is set to a value in the range of [1, CW+1], and for the
remaining contention rounds, the backoff counter is set to a value
in the range of [0, CW].
If it is determined in step 215 that a collision occurs in the
transmitted data signal, in step 221, the MS doubles a contention
window, starting from the first contention round to the
corresponding contention round determined by the number of
collisions. Then, returning to step 201, the MS sets the backoff
counter for each contention round, and repeats the subsequent
steps. If the collision occurs in a previously transmitted data
signal, it is determined that the collision occurs due to a
different MS having the same backoff counter. Thus, in order to
reduce an afterward collision probability, the contention window is
set to be greater than that in the previous time, thereby
increasing a possibility that the MS has a backoff counter
different from the different MS. According to an exemplary
embodiment of the present invention, a method is proposed in which,
when a j.sup.th collision occurs, a collision probability is
further reduced by approximately doubling a contention window,
starting from the first contention round to a j.sup.th contention
round.
The contention window can be increased by using Equation (1)
below.
.times..A-inverted..times..di-elect
cons.<.ltoreq..A-inverted..times..di-elect cons.<.ltoreq.
##EQU00001##
Herein, CW.sub.i denotes a contention window of an i.sup.th
contention round, and CW.sub.min denotes a minimum contention
window. In addition, j denotes the number of collisions, that is,
the number of times of attempting retransmission, and G denotes a
last contention round. For example, if the number of collisions is
2, contention windows of first and second contention rounds are
approximately doubled, and contention windows of the remaining
contention rounds are maintained without alteration.
Now, an operation of an MrCA method according to an exemplary
embodiment of the present invention will be described with
reference to FIG. 3. It will be assumed herein that three users (or
MSs) participate in a quadruple contention, and CW.sub.min of each
contention round is 3. In FIG. 3, a square box denotes a backoff
counter at each slot, and a shadowed square box denotes a backoff
counter of a current contention round.
First, a user 1, a user 2, and a user 3, after randomly setting
backoff counters, have backoff counter vectors respectively of
(3,2,2,0), (1,3,1,3), and (1,3,2,1). The first to fourth elements
included in each parenthesis respectively denote backoff counters
of first to fourth contention rounds. In the first contention
round, after one slot elapses, the users 2 and 3 complete counting.
At this time, each of the two users transmits a tone signal. Upon
receiving the tone signals from the two users, the user 1 waits for
a new contention in order to transmit next data while stopping the
counting in a state of (2,2,2,0). In the second contention round,
both the user 2 and the user 3 transmit the tone signal after three
slots elapse. In the third contention round, the user 2 transmits
the tone signal after one slot elapses, and thus the user 3, who
receives the tone signal, stops the counting in a state of
(0,0,1,1). In the fourth contention round, only the user 2
transmits data after three slots elapse. In this case, the user 2
may receive an ACKnowledge (ACK) signal as a response signal from a
receiving side.
In a new contention for transmitting subsequent data, the three
users again participate in the contention. The user 3, of which a
counter for the first and second contention rounds is zero from a
previous contention, continuously transmits a tone signal in a
current contention, thus stopping the counting of other users and
winning in the current contention.
FIG. 4 is a graph for comparing a collision rate with respect to
the number of contending nodes according to an exemplary embodiment
of the present invention with a conventional method.
Referring to FIG. 4, in comparison with an IEEE 802.11 Distributed
Coordinate Function (DCF), an exemplary Multi-round Collision
Avoidance (MrCA) method of the present invention can reduce a
collision rate to below 50%. In the graph of FIG. 4, G denotes the
number of contention rounds, and CW denotes a size of the
contention window. In addition, in FIG. 4, a single contention
(CW=31), in which one round contention is carried out for 32 slots,
a double contention (CW=15), in which the 32 slots are divided so
that 16 slots of two contention rounds are carried out, and a
quadruple contention (CW=7), in which the 32 slots are divided into
four contention rounds, are compared with one another. Although the
same 32 slots are used in all of the three cases, it can be seen
that collisions can be much more effectively avoided when the slots
are divided into several contention rounds. In particular, in the
quadruple contention, a collision rate remains at about 1% even
when the number of users reaches 50.
FIG. 5 is a graph for comparing a throughput with respect to the
number of contending nodes according to an exemplary embodiment of
the present invention in comparison with the conventional
method.
Referring to FIG. 5, an MrCA method according to an exemplary
embodiment of the present invention can improve a throughput by
approximately 25% in comparison with an IEEE 802.11 DCF. In the
graph of FIG. 5, the MrCA results from allocating a backoff counter
of a first contention round in the range of [0, CW.sub.min], and
the modified MrCA (mMrCA) results from allocating the backoff
counter of the first contention round in the range of [1,
CW.sub.min+1].
FIGS. 6A to 6C are graphs for comparing a throughput fairness index
for all nodes according to an exemplary embodiment of the present
invention in comparison with the conventional method.
Referring to FIGS. 6A to 6C, when throughputs of all nodes are
equally the same, the fairness index is 1. As the fairness index
approaches one, the system is considered to be fairer. A long
simulation duration provides results of long-term fairness and a
short simulation duration provides results of short-term fairness.
The experimentation results show that fairness is significantly
improved when using the MrCA of the present invention. In
particular, as the number of users increases, the increase in the
throughput becomes apparent for the short-term fairness.
According to exemplary embodiments of the present invention, data
is transmitted by using MrCA in a wireless network employing a
random access channel or a distributed access scheme. Therefore,
the number of contending nodes is exponentially decreased for each
contention round, resulting in a significant decrease in a
collision probability in addition to improving fairness on channel
use among users.
While the invention has been shown and described with reference to
certain exemplary embodiments thereof, it will be understood by
those skilled in the art that various changes in form and details
may be made therein without departing from the spirit and scope of
the invention as defined by the appended claims and their
equivalents. Therefore, the scope of the invention is defined not
by the detailed description of the invention but by the appended
claims and their equivalents, and all differences within the scope
will be construed as being included in the present invention and
their equivalents.
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